Schottky FET Fabricated With Gate Last Process

ABSTRACT

A method for forming a field effect transistor (FET) includes forming a dummy gate on a top semiconductor layer of a semiconductor on insulator substrate; forming source and drain regions in the top semiconductor layer, wherein the source and drain regions are located in the top semiconductor layer on either side of the dummy gate; forming a supporting material over the source and drain regions adjacent to the dummy gate; removing the dummy gate to form a gate opening, wherein a channel region of the top semiconductor layer is exposed through the gate opening; thinning the channel region of the top semiconductor layer through the gate opening; and forming gate spacers and a gate in the gate opening over the thinned channel region.

FIELD

This disclosure relates generally to the field of field effecttransistor fabrication.

DESCRIPTION OF RELATED ART

In order to be able to make integrated circuits (ICs), such as memory,logic, and other devices, of a higher integration density than iscurrently feasible, field effect transistor (FET) dimensions must bescaled down, as FETs are an important component of many ICs. A Schottkyjunction source/drain complementary metal-oxide-semiconductor (CMOS)field effect transistor (FET) is a viable option for thin-body devicesand sub-30 nanometer (nm) gate CMOS technology. Schottky FETs may haverelatively low parasitic resistance and gate-to-drain parasiticcapacitance, due to the lack of raised source/drain regions, as well asabrupt source/drain junctions. In particular, to fabricate a sub-15 nmFET in silicon-on-insulator (SOI) with good electrostatics and controlof the channel region, the semiconductor material in the channel regionmay need to be very thin, about 7 nm or less. However, achieving aprecise thickness at such small dimensions may be difficult; thevariation in the thickness of the semiconductor material may be about 2nm up or down.

The silicon (Si) thickness available for the silicide in source anddrain regions may be even less, and with greater variation, because ofthe Si loss during spacer reactive ion etching (RIE) and pre-silicidecleaning. Insufficient Si thickness raises silicide encroachment anddelemination problems during the fully silicided source/drain process,since the metal amount needed is determined by Si thickness in thesource/drain. The source and drain semiconductor material may be builtup to avoid these issues using epitaxial growth; however, it isdifficult to grow an epitaxial layer on a semiconductor layer that isabout 5 nm thick or less.

SUMMARY

In one aspect, a method for forming a field effect transistor (FET)includes forming a dummy gate on a top semiconductor layer of asemiconductor on insulator substrate; forming source and drain regionsin the top semiconductor layer, wherein the source and drain regions arelocated in the top semiconductor layer on either side of the dummy gate;forming a supporting material over the source and drain regions adjacentto the dummy gate; removing the dummy gate to form a gate opening,wherein a channel region of the top semiconductor layer is exposedthrough the gate opening; thinning the channel region of the topsemiconductor layer through the gate opening; and forming gate spacersand a gate in the gate opening over the thinned channel region.

In one aspect, a field effect transistor (FET) includes a semiconductoron insulator substrate, the substrate comprising a top semiconductorlayer; source and drain regions located in the top semiconductor layer;a channel region located in the top semiconductor layer between thesource region and the drain region, the channel region having athickness that is less than a thickness of the source and drain regions;a gate located over the channel region; and a supporting materiallocated over the source and drain regions adjacent to the gate.

Additional features are realized through the techniques of the presentexemplary embodiment. Other embodiments are described in detail hereinand are considered a part of what is claimed. For a better understandingof the features of the exemplary embodiment, refer to the descriptionand to the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Referring now to the drawings wherein like elements are numbered alikein the several FIGURES:

FIG. 1 illustrates an embodiment of a method of fabricate a Schottkysource/drain FET with a gate last process.

FIG. 2 illustrates an embodiment of a dummy gate formed on a substrate.

FIG. 3 illustrates an embodiment of the device of FIG. 2 after formationof source and drain regions.

FIG. 4 illustrates an embodiment of the device of FIG. 3 after formationof supporting material and removal of the dummy gate to form a gateopening.

FIG. 5 illustrates an embodiment of the device of FIG. 4 after thinningof the channel region through the gate opening.

FIG. 6 illustrates an embodiment of the device of FIG. 5 after formationof a spacer in the gate opening.

FIG. 7 illustrates an embodiment of the device of FIG. 6 after formationof a gate dielectric and gate metal layer in the gate opening.

DETAILED DESCRIPTION

Embodiments of a Schottky source/drain FET with a thinned channelregion, and a method of making a Schottky source/drain FET with a gatelast process, are provided, with exemplary embodiments being discussedbelow in detail. The semiconductor material in which the source, drain,and channel are formed may be relatively thick (in the range from about10 nm to about 20 nm thick in some embodiments), allowing for relativelyeasy formation of source and drain silicide regions. The portion of thesemiconductor material located in the channel region may be thinned toan appropriate channel thickness using a process that allows precisethickness control, such as a sequential ozone modified Huang cleaning.The finished channel region of the FET may be from about 0 nm to about 7nm thick in some embodiments.

FIG. 1 illustrates an embodiment of a method 100 of making a FET with athinned channel region. FIG. 1 is discussed with reference to FIGS. 2-7.Method 100 is a gate last FET fabrication process. In block 101, a dummygate 205 is formed on a top semiconductor layer 204 of a semiconductoron insulator substrate. The semiconductor on insulator substrateincludes bottom semiconductor layer 201, buried insulator layer 202, andtop semiconductor layer 204. Bottom semiconductor layer 201 and topsemiconductor layer 204 may include silicon (Si) in some embodiments,and buried insulator layer 202 may include buried oxide (BOX) in someembodiments. Shallow trench isolation (STI) regions 203 a-b are alsoformed in the substrate in buried insulator layer 202 and topsemiconductor layer 204. STI regions 203 a-b prevent electrical leakagebetween various FET devices located on the substrate, and may include adielectric material in some embodiments. Dummy gate 205 may be made froma nitride material or silicon germanium (SiGe) in some embodiments. Topsemiconductor layer 204 may be from about 10 nanometers to about 20nanometers thick in some embodiments.

In block 102, silicide source and drain regions 301 a-b are formed intop semiconductor layer 204, as shown in FIG. 3. Source and drainregions 301 a-b may be from about 10 nm to about 20 nm thick in someembodiments. In embodiments in which source and drain regions 301 a-bare silicide, the silicide may be formed using a self-aligned silicideprocess. In a self-aligned silicide process, first, a metal layer isformed over the portion of top semiconductor layer 204 in which thesource and drain regions 301 a-b are to be formed, on either side ofdummy gate 205. The metal layer may be nickel (Ni), nickel platinum(NiPt), platinum (Pt), cobalt (Co), or titanium (Ti) in someembodiments, and may be formed by plating or sputtering in someembodiments. The top semiconductor layer 204 and metal layer are thenrapid thermal annealed (RTA) to cause the metal layer to react with aportion of top semiconductor layer 204 to form silicide in the sourceand drain regions 301 a-b, and any unreacted portion of the metal layeris then removed, resulting in the device 300 shown in FIG. 3. Theunreacted portion of the metal layer may be removed by etching. Thesilicide may be a nickel silicide, nickel platinum silicide, platinumsilicide, cobalt silicide, or titanium silicide in some embodiments.Workfunction tuning of the source and drain regions 301 a-b may also beperformed in some embodiments. Workfunction tuning may include formationof segregated interfacial dopant layers (not shown) between the silicidethat comprises source and drain regions 301 a-b and top semiconductorlayer 204. The segregated interfacial dopant layers may be formed by anyappropriate process, including but not limited to implantation of sourceand drain regions 301 a-b with dopants and annealing to drive thedopants to the interfaces between source and drain regions 301 a-b andtop semiconductor layer 204, forming the segregated interfacial dopantlayers.

In block 103, a supporting material 401 a-b is formed over source anddrain regions 301 a-b and around dummy gate 205. Dummy gate 205 is thenselectively removed, resulting in device 400 as shown in FIG. 4.Supporting material 401 a-b may be any material that allows selectiveremoval of dummy gate 205 without removal of supporting material 401 a-band top semiconductor layer 204. Supporting material 401 a-b may includeoxide in some embodiments. Supporting material 401 may be formed bydepositing the supporting material over the device 300 of FIG. 3, andperforming chemical mechanical polishing (CMP) to expose the top ofdummy gate 205. After the top of dummy gate 205 is exposed by the CMP,dummy gate 205 is selectively removed, resulting in gate opening 402.Removal of dummy gate 205 exposes top semiconductor layer 204 throughgate opening 402.

In block 104, top semiconductor layer 204 is thinned through gateopening 402, resulting thinned channel region 501 as shown in FIG. 5.Thinned channel region 501 may be formed using a sequential Ozonemodified Huang cleaning process in some embodiments. In a sequentialHuang cleaning process, a surface of the material to be thinned isoxidized using ozone (O₃), and the oxidized portion of the material isthen removed using diluted hydrofluoric acid (HF). The oxidation andremoval steps may be repeated as many times as necessary to achieve thedesired thickness of thinned channel region 501. The sequential ozonemodified Huang cleaning process allows precise control of the finalthickness of the thinned material. Thinned channel region 501 may befrom about 0 nm thick to about 7 nm thick in some embodiments.

In block 105, a spacer 601 a-b is formed on supporting material 401 a-band source/drain regions 301 a-b inside the gate opening 402. Spacer 601a-b may be nitride in some embodiments. Spacer 601 a-b may be formed bydeposition of the spacer material on the interior of gate opening 402,and then etching the spacer material to form spacer 601 a-b havingdesired dimensions. Any spacer material that that forms on the surfaceof thinned channel region 501 during deposition may also be removed bydirectional etching. The etching may include a dry etch such as reactiveion etching in some embodiments.

In block 106, a gate stack, including a gate dielectric layer 701 andgate metal 702, is formed in gate opening 402, resulting in a FET 700with a thinned channel region 501 as shown in FIG. 7. Gate dielectriclayer 701 is formed over spacer 601 a-b and thinned channel region 501,and may include a high k dielectric material in some embodiments. Gatemetal 702 acts as an electrical contact for the gate, and is formed overgate dielectric layer 701. FET 700 includes a relatively thin channelregion 501, from about 0 nm to about 7 nm thick in some embodiments, andrelatively thick source and drain regions 301 a-b, from about 10 nmthick to about 20 nm thick in some embodiments.

The technical effects and benefits of exemplary embodiments includefabrication of a scaled FET with a relatively thin channel region andrelatively thick source and drain regions, allowing for good control ofthe channel region.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an”, and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

1. A method for forming a field effect transistor (FET), the method comprising: forming a dummy gate on a top semiconductor layer of a semiconductor on insulator substrate; forming source and drain regions in the top semiconductor layer, wherein the source and drain regions are located in the top semiconductor layer on either side of the dummy gate; forming a supporting material over the source and drain regions adjacent to the dummy gate; removing the dummy gate to form a gate opening, wherein a channel region of the top semiconductor layer is exposed through the gate opening; thinning the channel region of the top semiconductor layer through the gate opening; and forming gate spacers and a gate in the gate opening over the thinned channel region.
 2. The method of claim 1, wherein the semiconductor on insulator substrate comprises an insulator layer underneath the top semiconductor layer, and a bottom semiconductor layer under the insulator layer.
 3. The method of claim 2, wherein the top semiconductor layer and the bottom semiconductor layer comprise silicon, and the insulator layer comprises buried oxide.
 4. The method of claim 1, wherein the source and drain regions comprise silicide.
 5. The method of claim 1, wherein the supporting material comprises oxide.
 6. The method of claim 1, wherein forming the supporting material comprises: depositing the supporting material over the dummy gate and source and drain regions; and chemical mechanical polishing a top surface of the deposited supporting material to expose a top surface of the dummy gate.
 7. The method of claim 1, wherein thinning the channel region comprises sequential ozone modified Huang cleaning.
 8. The method of claim 1, wherein the dummy gate comprises one of nitride and silicon germanium.
 9. The method of claim 1, wherein forming the gate spacers and the gate comprises: depositing a spacer material in the gate opening; etching the spacer material to form the gate spacers; forming a gate dielectric layer in the gate opening over the gate spacers and the channel region; and forming a gate metal layer in the gate opening over the gate dielectric layer.
 10. The method of claim 9, wherein the spacer material comprises nitride, wherein etching the spacer material comprises reactive ion etching, and the wherein the gate dielectric layer comprises a high k dielectric material.
 11. The method of claim 1, wherein the thinned channel region is from about 0 nanometers thick to about 7 nanometers thick.
 12. The method of claim 1, wherein the source and drain regions are from about 10 nanometers thick to about 20 nanometers thick.
 13. A field effect transistor (FET), comprising: a semiconductor on insulator substrate, the substrate comprising a top semiconductor layer; source and drain regions located in the top semiconductor layer; a channel region located in the top semiconductor layer between the source region and the drain region, the channel region having a thickness that is less than a thickness of the source and drain regions; a gate located over the channel region; and a supporting material located over the source and drain regions adjacent to the gate.
 14. The FET of claim 13, wherein the semiconductor on insulator substrate comprises an insulator layer underneath the top semiconductor layer, and a bottom semiconductor layer under the insulator layer.
 15. The FET of claim 13, wherein the source and drain regions comprise silicide.
 16. The FET of claim 13, wherein the supporting material comprises oxide.
 17. The FET of claim 13, further comprising gate spacers formed on the supporting material, and wherein the gate comprises a gate dielectric layer formed over the gate spacers and the channel region, and a gate metal layer formed over the gate dielectric layer.
 18. The FET of claim 17, wherein the gate spacers comprise nitride and the gate dielectric layer comprises a high k dielectric material.
 19. The method of claim 1, wherein the thinned channel region is from about 0 nanometers thick to about 7 nanometers thick.
 20. The method of claim 1, wherein the source and drain regions are from about 10 nanometers thick to about 20 nanometers thick. 